Ryznar Stability Index(RSI)

Ryznar Stability Index(RSI)

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What is Ryznar Stability Index?

Ryznar Stability Index(RSI) was developed by John Ryznar in 1940. It is also called Rznar Index.

The reason behind the development of ryznar index is to get more accurate prediction of calcium carbonate scale than Langalier Saturation Index which was developed in 1936.

Ryznar Index has its own concept & basis of prediction. However the calculation of saturation pH is same as LSI but the difference is in the formula & prediction values.

In Ryznar Index, any value above 6 indicates the water is likely to form calcium carbonate scale while on the other hand any value below 6 water will dissolve calcium carbonate scale and considered the tendency of water is corrosive.

RSI Prediction
RSI <5.5; Heavy scale will form
5.5< RSI <6.2; Moderate scale will form
6.2< RSI <6.8; Slight scale will form
6.8< RSI <8.5; Corrosive water
RSI <8.5; Very corrosive water

Ryznar Stability Index Calculator:

Ryznar Stability Index Calculation Formula:

RSI Formula

A =( LOG10 * (TDS)-1) / 10

B = (-13.12 * LOG10(temp +273)) + 34.55

C = LOG(CaH) – 0.4

D = LOG10(M-Alk)

pHs = (9.3 + A + B) – (C + D)

RSI = 2pHs – pHa

Where;

TDS = Total Dissolved Solids(ppm)

temp = Temperature(dEG C)

CaH = Calcium Hardness(ppm as CaCO3)

M-Alk = M-Alkalinity(ppm as CaCO3)

pHs = Saturation pH

pHa = Actual pH

Ryznar Index Calculation Example:

pH 8.0
Total Dissolved Solids 500 mg/L
Temperature 30 dEG C
Calcium Hardness 250 ppm as CaCO3
M-Alkalinity 100 ppm as CaCO3

Lets Calculate..

Put cooling water analysis values in above Formula to calculate RSI. You get below values for constants A,B,C & D as per below table:

A 0.17
B 2.02
C 2.0
D 2.0
pHs 7.49

Finally, RSI = 2pHs – PHa

 RSI = (2 * 7.49) – 8.0 = 6.98 

The tendency of this water is slightly scale forming.

 

 

Cooling Tower Calculations

Cooling Tower Calculations

In order to understand cooling tower calculations, you need to understand some basic terminology & formulas.

Let’s start…

Cooling Tower Calculations & Terminologies:

#1. Wet bulb temperature:

wet bulb temperature is measured by the thermometer which is wrapped in a cloth called soak. Wet bulb temperature of a cooling tower is measured by sling psychomotor.

#2. Dry bulb temperature:

This the temperature of the atmosphere. It is also called ambient temperature. It doesn’t take account of relative humidity in the air. Relative humidity simply represents how much moisture could be at a given temperature compared to the actual moisture present in the air. If the humidity is 100% then no evaporation is possible because air is completely saturated with water.

#3. Range or Delta T:

It is the difference between cooling water inlet temperature and outlet temperature.

Range or Delta T Calculation
Range or Delta T = Hot cooling water inlet temp – Cold cooling water outlet temp

#4. Approach:

This is the difference between the cooling tower outlet cold water temperature and ambient wet bulb temperature.

Approach Calculation
Approach = Cold cooling water outlet – Wet bulb temperature

#5. Cooling Tower Effectiveness:

This is the ratio of range to the ideal range

CT Effectiveness Calculation
CT effectiveness (%) = Range / (Range + Approach) *100

#6. Hold up volume:

It is the total volume of water present in the whole circuit of the cooling tower including piping & equipment. Don’t confuse with circulation rate. The holdup volume is measured in m3

#7. Circulation Rate or Re-circulation Rate:

It is the flow rate of water which is circulated in the cooling tower. Normally, the circulation rate is measured in m3/hr

#8. Evaporation Loss:

Evaporation Loss: It is the loss of water from a cooling tower by evaporation. Theoretically, the evaporation quantity of water is 1.8 m3 for every 10,00,000 Kcal heat rejected.

Evaporation Loss Calculation
Evaporation Loss(m3/hr) = 0.00153 * Recirculation Rate (m3/hr)  * Delta T

#9. Windage or Drift Loss:

It is very difficult to ignore the drift problem in a cooling tower. Drift or windage loss of cooling tower is normally provided by its manufacturer based on cooling tower design. If it isn’t available then you can assume based on below formula.

Drift Loss Calculation
Natural Draft Cooling Tower: 0.3 to 1.0 * Recirculation Rate / 100

Induced Draft Cooling Tower: 0.1 to 0.3 * Recirculation Rate / 100

Cooling Tower with drift eliminator: 0.01 * Recirculation Rate / 100

#10. Cycle of concentration(COC):

cycle of concentration (COC): It is simply a ratio of the parameters of cooling water to the parameters of makeup water. It is a dimensionless number. It can be calculated by any of the below formulae.

COC Calculation
COC = Silica in cooling water / Silica in makeup water

COC = Calcium Hardness in cooling water / Calcium Hardness in makeup water

COC = Conductivity in cooling water / Conductivity in makeup water.

COC = Make up water quantity / Blowdown water quantity

The last formula gives you more accurate COC if you have flow measurement facility available for makeup & Blowdown water in the cooling tower. The cycles of concentration normally vary from 3.0 to 8.0 depending on the design of a cooling tower.

It is always advisable to maintain COC as high as possible to reduce make water requirement. It is ultimately saves the water. On other side higher COC increases dissolved solids concentration in cooling tower.

#11. Blow down:

As you know when water evaporates it leaves solids & only pure water evaporates. It means as COC increases dissolved solids gets concentrate. This will lead to corrosion & scaling problem in the system if COC is not maintained as per design limit. So, to maintain design COC some quantity of water is discharged from the cooling tower. It is known as Blow down & calculated based on below formula

Blowdown Calculation
Blow down = Evaporation Loss / COC-1

#12. Holding Time Index:

It is a measurement of time at which the concentration of the added chemical into the cooling water system decreases to 50% of its original value. This happens due to blowdown & drift loss of water from the system plus the addition of new makeup water in the system. The ideal value for HTI is 24 hours. High HTI (>48 hours) can result in chemical degradation.

HTI Calculation
HTI = 0.693 * Hold up Volume / Blowdown

#13. Chemical dosing calculation based on blowdown:

Chemicals like corrosion and scale inhibitors are dosing on a continuous basis so the dosing of these chemicals is calculated based on blowdown rate. Basically, the purpose is to make up the chemical which is lost with blowdown to maintain desired concentration. This calculation is a very important part of any cooling tower calculations.

Chemical Calculation Based on Blowdown
Chamical Quantity (Kg/hr) = Blowdown (m3 /hr) * ppm / 1000

#14. Chemical dosing calculation based on holdup volume:

Slug dosed chemicals like non-oxidising biocide calculated based on hold up volume of cooling tower. Generally, after dosing of biocide blowdown is closed for 24 hours to make it more effective.

Chemical Calculation Based on Holdup
Chamical Quantity (Kg) = Hold up volume (m3) * ppm / 1000

#15. Langelier Saturation Index:

LSI calculation will indicate the calcium carbonate scaling tendency of the water. Calculating an LSI is very important because exceeding a treatment program’s LSI limit will likely lead to the formation of a calcium carbonate deposit.

#16. Log Mean Temperature Difference(LMTD):

This calculates average temperature differential across heat exchangers. It compares the difference between temperatures of the “hot” and “cold” fluids in heat exchangers. The larger temperature difference between two fluids at either the exit or the entrance of the heat exchanger is designated as ∆T2 and the smaller temperature difference is designated as ∆T1

Counterflow heat exchanger design where hot fluid enters at the opposite side of cooling water. The LMTD of counterflow heat exchanger design calculated by using the below formula:

LMTD Calculation For Counter flow Heat Exchanger
LMTD = [(T1 – t2) – (T2 – t1)] / ln [(T1 – t2) – (T2 – t1)]

Parallel flow heat exchanger design where hot fluid & cooling water enters on the same side of the heat exchanger. The LMTD of parallel flow heat exchanger design calculated by using the below formula:

LMTD Calculation For Parallel flow Heat Exchanger
LMTD = [(T1 – t1) – (T2 – t2)] / ln [(T1 – t1) – (T2 – t2)]
Where;

T1 = Hot fluid inlet temperature

T2 = Hot fluid outlet temperature

t1 = Cold fluid inlet temperature

t2 = Cold fluid outlet temperature

#17.Terminal Temperature Difference(TTD):

TTD is the difference in temperatures of hot fluid exiting (T hot-out ) and cold fluid exiting (T cold-out ) the heat exchanger.

TTD Calculation
TTD = T hot(out) – T cold(out)

Why LMTD & TTD is important?

Increasing LMTD & TTD means there is reduced heat transfer occurring, and the system might be fouling on the process side or the cooling water side.

In Conclusion, above all cooling tower calculation are the most important part of any cooling water treatment program to monitor it very effectively.

 

Cooling Tower Components & Functions

Cooling Tower Components & Functions

The main components of cooling tower are: Frames, fill, nozzles, drift eliminators, fan, air inlet, louvers, cold water basin.

Let us understand the definition & functions of each of them one by one.

Cooling Tower Components:

components of cooling tower

Frame: Frame is basically a structure of cooling tower that holds all essential components of cooling tower like fan, motors etc.

Fill: Normally, Fills are made of plastic or wood. Fill can either be a splash type or film type.

In splash fill water falls over horizontal layers of splash bars that continuously breaks water into small droplets and also wetting the fill surface. The rate of heat transfer in plastic splash fill is better that wood splash fill.

Film fill consists of thin closely placed plastic surfaces on them water spreads and form a thin film in contact with air. These surfaces may be flat, honeycombed or other patterns. Film fill is more efficient than splash fill.

Nozzles: The function of nozzles is to spray water and wet the fill. Uniform water distribution is essential to achieve proper wetting of entire fill surface. Nozzles can either be fixed in place and have either round or square spray patterns or can be part of a rotating assembly as found in some circular cross-section towers.

Drift Eliminators: It captures water droplets that escape with air from the tower. Otherwise water would be lost to atmosphere. Ultimately drift eliminators save loss of water from cooling tower.

Fans: axial (propeller type) and centrifugal fans are used in towers. Generally, propeller fans are used in induced draft tower while both propeller & centrifugal fans are used in forced draft towers.

Air Inlet: This is the entry point of air into the cooling tower. This entry point may be entire side of a tower (cross section design) or bottom of the tower (counter flow design).

Louvers: The main purpose of the louvers is to equalize the air flow into the fill & retain water into the tower. Generally, cross flow towers has inlet louvers while many counter flow tower designs do not require louvers.

Cold Water Basin: The cold water basin located at near or bottom of the tower. The function of cold water basin is to receive cold water which is flow through the fills. The basin usually has a sump or low point for the cold water discharge connection.

Cooling Tower Material

Earlier, cooling towers were constructed primarily with wood, including the frame, casing, louvers, fill and cold-water basin. In some cases the cold-water basin was made of concrete.

Today, manufacturers are using a variety of materials to construct cooling towers. Materials are chosen based on properties like enhance corrosion resistance, reduce maintenance, and promote reliability and long service life.

Galvanized steel, various grades of stainless steel, glass fiber, and concrete are commonly used in construction of cooling tower. Aluminum and plastics are also use for some components.

#1. Frame and Casing: You can still found wooden cooling towers, but many components of this tower made from different materials, such as the casing around the wooden framework made of glass fiber, the inlet air louvers of glass fiber, the fill of plastic and the cold-water basin of steel.

Where corrosive atmosphere is an issue galvanized steel is use to construct casings & basins of the tower.

Mostly, larger towers are made of concrete. Glass fiber is also broadly used for cooling tower casings and basins, because they extend the life of the cooling tower by protecting cooling tower from harmful chemicals.

#2. Fill: Plastics are generally used for fill, including PVC, polypropylene, and other polymers. Film fill is the first choice because of its higher heat transfer efficiency where the circulating water is free from debris or contains less suspended solids.

#3. Nozzles: Plastics are commonly used for nozzles. Many nozzles are made of PVC, ABS, polypropylene, and glass-filled nylon.

#4. Fan: Aluminum, glass fiber and galvanized steel are mostly used as fan materials. Centrifugal fans are often made from galvanized steel. Propeller of fans are made from galvanized steel, aluminum or molded glass fiber reinforced plastic.

Types of Cooling Tower

Types of Cooling Tower

in this article, I addresses both the important question what is cooling tower & how many types of cooling tower?

Are you ready to learn?

Let’s start..

What is Cooling Tower?

Cooling tower is simply a one type of heat exchange equipment where air & water comes in to the direct contact with each other. The main function of cooling tower is to reject heat of hot water into atmosphere & cooling down the water for further reuse.There are different types of cooling tower

Evaporation of water is responsible for majority of heat rejected from water in cooling tower. Typically 75% to 80% of heat is removed from water by evaporation process. Remaining heat is removed by passing airflow through the cooling tower.  

Cooling Tower is also a water saver equipment where you can save huge amount of water by recycling the water. Open recirculation system is very popular in all types of industries because of water recycling feature.

In most of the industrial process,water is widely used as a cooling medium because it is very efficient,readily available & relatively cheap.

Types of Cooling Towers:

Cooling towers are mainly divide in two categories.

  1. Natural Draft Cooling
  2. Mechanical Draft

types of cooling tower

Let us discuss each of them in detail.

Natural Draft Cooling Tower:

Natural draft cooling towers use very large concrete chimneys to introduce air through the media. Warm, moist air naturally rises due to the density difference compared to outside air which is cool & dry.

Hyperbolic shape of tower drive moist air upwards (buoyancy) & pulls cold air into the tower. Due to the large size of these towers typically 350 to 500 feet, they are generally used for water flow rates above 45,000 m3/hr. usually, large utility power stations uses these types of towers.

natural draft cooling tower

The principle is same as mechanical draft tower but here fan unit is missing because Heat is removed from water by natural draft. This type of tower reduce both operating & energy consumption cost due to its natural draft given by height & stack dimension. Additionally, natural draft cooling tower has a long service life, low noise emissions & low maintenance.

Mechanical Draft Cooling Tower:

Mechanical draft cooling towers utilize power driven motor fans to force or draw air through the tower. These large fans force or suck air through circulated water.

The water then falls downward over fill surfaces, which helps increase the contact time between the water and the air and maximizes heat transfer between them.

Cooling rates of Mechanical draft towers depend upon their fan diameter and speed of operation.

Mechanical draft towers are available in the following airflow arrangements:

  • Induced draft
  • Forced draft

Induced Draft Cooling Tower:

An Induced draft cooling tower is a mechanical draft tower with a fan at the discharge to pull air through it. The fan induces hot moist air out the discharge.

This produces low entering and high exiting air velocities, reducing the possibility of recirculation in which discharged air flows back into the air intake.This fan/fill arrangement is also known as “Draw-through”.

induced draft counter flow cooling tower

This is the diagram of counter flow induced draft design. in this design hot water enters at the top & air enters from the bottom and exit at the top of the tower.

induced draft cross flow cooling tower design

This is the diagram of induced draft cross flow cooling tower. In this arrangement water enters from the top and passes over the fill but here the difference is in the air inlet path. Air is enters form the side of the tower as you can see in the diagram. An induced draft fan draws air across the wet fill and expels it through the top of the structure.

Forced draft:

A Forced draft cooling tower is a mechanical draft tower with a blower type fan at the intake. The fan forces air into the tower, creating high entering and low exiting air velocities.

The low exiting velocity is much more susceptible to recirculation. With the fan on the air intake, the fan is more susceptible to complications due to freezing conditions.

Another disadvantage is that a forced draft design typically requires more motor horsepower than an equivalent induced draft design.The forced draft benefit is its ability to work with high static pressure. They can be installed in more confined spaces and even in some indoor situations.

This fan/fill geometry is also known as “Blow-through.

Based on the movement of Air and the Hot Water Flow these can be classified into two types:

  • Cross flow
  • Counter flow

Cross Flow Cooling Tower:

Cross flow is a design in which the air flow is directed perpendicular to the water flow. Air flow enters one or more vertical faces of the cooling tower to meet the fill material.

Water flows (perpendicular to the air) through the fill by gravity. The air continues through the fill and thus past the water flow into an open plenum area.

A distribution or hot water basin consisting of a deep pan with holes or nozzles in the top is utilized in a cross flow tower.

Gravity distributes the water through the nozzles uniformly across the fill material.

Counter Flow Cooling Tower:

In a counter flow design the air flow is directly opposite to the water flow. Air flow first enters an open area beneath the fill media and is then drawn up vertically.

The water is sprayed through pressurized nozzles and flows downward through the fill, opposite to the air flow.

Both Cross flow and Counter flow designs can be used in Mechanical draft cooling towers.

Hope, this article is clear your concept of what is cooling tower & types of cooling towers. 

 

3 Types of Cooling System

3 Types of Cooling System

The selection of a types of cooling system is basically depends on the quantity & quality of the water source which is available. Each of these systems has it’s own advantages & disadvantages.The Three main types of cooling water system are:

  1. Once Through Cooling System
  2. Closed Recirculating System
  3. Open Recirculating System

Let’s discuss one by one in detail:

# 1. Once Through Cooling System:

Once through system means water circulates in the system only once & then discharged from the system. It means cooling water passes through the heat exchangers only once.

This system is only applicable where plenty of water is available. This is the major drawback of this system.

The possibility of chemical treatment is also limited because all added chemical discharged from the system after completion of first cycle which is environmentally & economically not favorable. Normally, only chlorination is helpful to this type of the system for microbiological control.

types of cooling system-once through

Pros & Cons:

Once Through Cooling System

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Pros
  • Low Capital Cost
  • Low Operating Cost
  • Low Temperature Scale

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Cons
  • Discharge of large quantity of hot water
  • Absence of adequate chemical control for scaling

# 2. Closed Recirculating Cooling System:

In closed recirculating system cooling water is circulates in closed loop means within the system pipes and heat exchangers. The heat absorbed from the plant process is generally dissipated by air-cooling.

Loss from the system is very less and little make up is required, hence concentration of salt in the water is very less.

Deposition /scale formation is not a problem in this system. Major problem may be corrosion, which can be easily controlled by use of suitable corrosion inhibitor.Generally,Nitrite based inhibitors are very effective.

Types of cooling system closed recirculating

Pros & Cons:

Closed Recirculating System

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Pros
  • Very limited make-up required
  • Small volume of chemical required to treat
  • Scaling is not a problem because of high quality make up water
  • Less susceptible to biological fouling

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Cons
  • Only economically applicable for small cooling system
  • Requires another cooling system to run this system
  • Expense of additional heat exchangers

# 3. Open Recirculating Cooling System:

This is the most popular & widely used system in almost all industries. This type of system is applicable where water supply is restricted or hot effluent can’t be tolerated for environmental reasons.The good quality of water requires for this system to allow recycling of water.

When water evaporates dissolved solids present in water don’t evaporates but they gradually concentrates. This will lead to corrosion & scaling problems in the system. It can control by cooling water treatment programs. Generally, the heat exchanger & piping system are made of carbon steel.

In this system water recirculates according to design cycles of concentration which is also known as COC.

Chemical treatment is essential for this system. You can increase cycles of concentration of cooling tower with the help of proper treatment programs. You can also save large amount of water by operating cooling tower on higher COC.

types of cooling system open recirculating

Pros & Cons:

Open Recirculating System

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Pros
  • Saves large amount of water
  • Reduce discharge rate of water
  • Chemical control is possible

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Cons
  • Higher capital cost
  • Higher operating cost

In conclusion, Open recirculating system is best choice because it saves large amount of water & you can also control scaling & corrosion by applying suitable chemical treatment.

Closed system requires additional cooling system this is the major disadvantage & only applicable for small systems while on the other hand to operate once through cooling system plenty of water is required & Chemical control is also not possible.

 

Langelier Saturation Index

Langelier Saturation Index

What is Langelier Saturation Index?

Langelier Saturation Index(LSI) was developed by Scientist Langelier in 1936.

The LSI is used to determine the need for calcium carbonate precipitation scale control in water sources containing a TDS concentration of less than 10,000 mg/L

LSI is most widely used tool to predict tendency of cooling water. In order to calculate LSI five values namely TDS(Total Dissolved Solid), Temperature, Calcium Hardness, M-Alkalinity & pH are  very important.

Based on these five values you can calculate saturation pH called pHs & then subtract it from actual pH to get LSI.

Actually, LSI predicts tendency of CaCO3(Calcium Carbonate), whether it will precipitate it or dissolve it.

Basically,This information is important at the time of designing a cooling water chemical treatment program, especially in water that have a tendency to form calcium carbonate scale.

LSI Prediction

If LSI is Negative; No potential to scale,Water will dissolve CaCO3
If LSI is Positive; Scale will form & CaCO3 precipitation occurs
If LSI is ZERO; Water is considered as stable water

Langelier Saturation Index Calculator:

LSI Potential For CaCO3 Deposition
3.0 Extremely Severe
2.0 Very Severe
1.0 Severe
0.5 Moderate
0 Stable Water
-0.5 Non-scaling,Slight tendency to dissolve CaCO3
-1.0 Non-scaling,Moderate tendency to dissolve CaCO3
-2.0 Non-scaling,Strong tendency to dissolve CaCO3
-3.0 Non-scaling,Very strong tendency to dissolve CaCO3

LSI Calculation Formula:

LSI Formula

A =( LOG10 * (TDS)-1) / 10

B = (-13.12 * LOG10(temp +273)) + 34.55

C = LOG(CaH) – 0.4

D = LOG10(M-Alk)

pHs = (9.3 + A + B) – (C + D)

LSI = pHa – pHs

Where;

TDS = Total Dissolved Solids(ppm)

temp = Temperature(dEG C)

CaH = Calcium Hardness(ppm as CaCO3)

M-Alk = M-Alkalinity(ppm as CaCO3)

pHs = Saturation pH

pHa = Actual pH

LSI Calculation Example:

pH 8.0
Total Dissolved Solids 500 mg/L
Temperature 30 dEG C
Calcium Hardness 250 ppm as CaCO3
M-Alkalinity 100 ppm as CaCO3

Lets Calculate..

Put cooling water analysis values in above LSI Formula. You get below values for constants A,B,C & D as per below table:

A 0.17
B 2.02
C 2.0
D 2.0
pHs 7.49

Finally, LSI = pHa – PHs

 LSI = 8.0 – 7.49 = 0.51 

The tendency of this water is slightly scale forming.

 

 

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